This invention relates generally to color cathode ray tubes (CRTs) and is particularly directed to the control of multiple electron beams incident upon the faceplate of a color CRT.
Most color CRTs employ an inline electron gun arrangement for directing a plurality of electron beams on the phosphorescing inner screen of its glass faceplate. The inline electron gun approach offers various advantages over earlier "delta" electron gun arrangements particularly in simplifying the electron beam positioning control system as well as essentially eliminating the tendency of the electron beams to drift. However, inline color CRT's employ a self-converging deflection yoke which applies a nonuniform magnetic field to the electron beams, resulting in an undesirable astigmatism in and defocusing of the electron beam spot displayed on the CRT's faceplate. In order to achieve three electron beam convergence at the screen edges and corners, the self-converging yoke applies a dynamic quadrupole magnetic field to the beams which over-focuses the beams in the vertical direction and under-focus them in the horizontal direction. This is an inherent operating characteristic of the inline yoke design.
One approach to eliminate this astigmatism and deflection defocus employs a quadrupole lens with the CRT's focusing electrode which is oriented 90.degree. from the self-converging yoke's quadrupole field. A dynamic voltage, synchronized with electron beam deflection, is applied to the quadrupole lens to compensate for the astigmatism caused by the deflection system. This dynamic voltage also allows for dynamic focusing of the electron beams over the entire CRT screen. The astigmatism of the electron beam caused by the quadrupole lens tends to offset the astigmatism caused by the color CRT's self-converging deflection yoke and generally improves the performance of the CRT.
In order to achieve the three beam dynamic convergence at the screen corners, the yoke fields over focus the beams in the vertical direction and under focus them in the horizontal direction. To a simplified model, the self-convergence inline yoke's magnetic fields can be represented by a uniform two pole magnetic field plus a quadrupole magnetic field, as shown in FIG. 1, for both horizontal and vertical deflection fields. This is an inherent property of the inline yoke design and there is no trade-off (in yoke design alone) making it possible to achieve both the three-beam self-convergence plus good edge and corner focus at the same time.
Basically, these dynamic quadrupole designs use a split focus grid either 2 parts (bipotential) or 3 parts (Einzel). On these split focus grids some type of electrostatic quadrupole shaped grid design is used. When there is a voltage difference between the split grids, an electrostatic quadrupole field is formed. The strength and timing of such an electrostatic quadrupole can be controlled to cancel the inline yoke's undesirable negative quadrupole effect, and improve the spot size performance over both the center and corners. In recent years some manufacturers have further proposed using a uniform field yoke plus a separate magnetic quadrupole coil to achieve homogeneous spot performance over the whole screen.
An article entitled "Progressive-Scanned 33-in. 110.degree. Flat-Square Color CRT" by Suzuki et al published in SID 87 Digest, at page 166, discloses a dynamic astigmatism and focus (DAF) gun wherein spot astigmatism and deflection defocusing is simultaneously corrected using a single dynamic voltage. The electron gun employs a quadrupole lens to which the dynamic voltage is applied and which includes a plurality of generally vertically elongated apertures in a first section of a focusing electrode and a second pair of aligned, generally horizontally oriented elongated apertures in a second section of the focusing electrode. Each electron beam first transits a vertically aligned aperture, followed by passage through a generally horizontally aligned aperture in the single quadrupole lens for applying astigmatism correction to the electron beam.
An article entitled "Quadrupole Lens For Dynamic Focus and Astigmatism Control in an Elliptical Aperture Lens Gun" by Shirai et al, also published in SID 87 Digest, at page 162, discloses a quadrupole lens arrangement comprised of three closely spaced electrodes, where the center electrode is provided with a plurality of keyhole apertures and the outer electrodes are provided with a plurality of square recesses each with a circular aperture in alignment with each of the respective electron beams. A dynamic voltage V.sub.d is applied to the first and third electrodes so as to form a quadrupole field to compensate for the astigmatism caused by the self converging yoke deflection system. Although this allows for a reduction in the dynamic voltage applied to the quadrupole, this voltage still exceeds 1 KV in this approach. While these two articles describe improved approaches for beam focusing and astigmatism compensation, they too suffer from performance limitations particularly in the case of those CRTs having a flat faceplate and foil tension shadow mask, where the flat geometry imposes substantially greater challenges than those encountered with a curved faceplate.
An electron gun employing a quadrupole lens to which a dynamic voltage is applied generally also includes a Beam Forming Region (BFR) refraction lens design intended to correct for the lack of dynamic convergence of the red and blue outer electron beams. The horizontal beam landing locations of the red and blue beams in color CRTs having an inline electron gun arrangement change with variations in the focus voltage applied to the electron gun. While the dynamic quadrupole lens compensates for astigmatism caused by the self-converging electron beam deflection yoke, prior art quadrupole lens arrangements do not address the lack of horizontal convergence of the two outer electron beams.
In a more general sense, this invention addresses the problem of how to electrically converge off-axis beams in a three-beam color cathode ray tube, particularly a color cathode ray tube of the type having an inline gun.
There exists a number of techniques in the prior art for electrically converging off-axis electron beams in a color cathode ray tube. One technique offsets the axes of apertures in facing electrodes. Offsetting the axes of the cooperating apertures creates an asymmetrical field which bends an electron beam in a direction dependent upon the asymmetry and strength of the field. Examples of electron guns having such offset-aperture-type beam bending are U.S. Pat. Nos. 3,772,554; 4,771,216 and 4,058,753.
A second approach is to use coaxial apertures, but angle the gap between the facing electrodes to produce the necessary asymmetrical field. Examples of electron guns having such "angled gap" technique for producing the necessary asymmetrical field are disclosed in U.S. Pat. Nos. 4,771,216 and 4,058,753.
A third approach is to create the asymmetrical field for the off-axis beam or beams by creating a wedge-shaped gap between the addressing electrodes. Examples of this third approach for electrically converging off-axis beams are disclosed in U.S. Pat. Nos. 3,772,554 and 4,058,753.
Each of these three approaches suffers from difficulties in mandrelling the electrodes during assembly. One aspect of the present invention is to provide improved means in an electron gun for refracting or bending an electron beam, useful for converging off-axis beams in a color CRT gun.
As discussed above, certain modern high performance electron guns have a dynamic quadrupole lens to compensate for beam astigmatism introduced by an associated self-converging yoke. The aforementioned U.S. Pat. No. 4,771,216 discloses the use of a dynamic quadrupole lens for providing a dynamic astigmatism correction for an inline electron gun having separate aligned sets of apertures for each of the three electron beams. The disclosure and discussion of the prior art set forth in the '216 patent are relevant to the present invention and are hereby incorporated by reference in this application.
The '216 patent discloses a gun system of the type in which static convergence is achieved by creating asymmetrical fields in the paths of the off-axis beams, which asymmetrical fields can be created using offset apertures, wedged interelectrode gaps or angled gaps. In addition, a balanced quadrupole is utilized to provide astigmatism correction. The quadrupole and the means for creating the aforesaid asymmetric fields for convergence are separate. Each of the beams are focused using main focus fields which are discrete for each of the three beams.
Co-pending application Ser. No. 521,505 discloses an electron gun system having a self-converging yoke, focusing means of the type in which changes in focusing field strength alters beam convergence, and an unbalanced quadrupole. The quadrupole is provided for astigmatism correction. Application of a dynamic waveform to the astigmatism corrector has the undesired effect of producing dynamic convergence in the beam focusing means. Since full convergence is provided by the self-converging yoke, the undesired beam convergence produced by the focusing means represents convergence errors. In accordance with the invention described and claimed therein, the astigmatism-correcting quadrupole is caused to be unbalanced in a sense such as to offset the convergence errors produced by the focusing means.
There exists a need for a cathode ray tube system in which all or a major part of the beam convergence is achieved in the electron gun system. With such a gun system, self-convergence demands on the yoke may be reduced or eliminated entirely. The astigmatism of the beams inevitably produced by the self-converging yoke would thus be reduced or eliminated completely.